git-gui: allow larger width for the commit message field

git-gui: allow larger width for the commit message field

Users may like to make the main window very wide. In this case, a rather
small size of the commit message field leaves a wide unused space at the
bottom-right. Allow settting the field width to values much larger than
99 characters. In fact, users can set it to extreme values to fill the
entire space regardless of window width: the widget geometry will be
negotiated to be limited to the available space despite the large
requested width.

Signed-off-by: Alexander Shopov <redacted>